Allocation and retention priority was introduced into 2G/3G packet network according to TS 3GPP 23.107. It is used to specify the relative importance compared to other UMTS bearers for allocation and retention of the UMTS bearer. Priority is used for differentiating between bearers when performing allocation and retention of a bearer. In situations where resources are scarce, the relevant network elements can use the Allocation/retention Priority to prioritize bearers with a high Allocation/retention Priority over bearers with a low Allocation/Retention Priority when performing admission control.
The Allocation/Retention Priority attribute is a subscription attribute which is not negotiated from the mobile terminal, but the value might be changed either by the SGSN or the GGSN network element.
By introducing new services into packet domain such as multiple priority service, the Allocation and Retention Priority is evolved.
The evolved ARP comprises information about the priority level (scalar), the pre-emption capability (flag), and the pre-emption vulnerability (flag). The priority level defines the relative importance of a resource request. This allows deciding whether a bearer establishment or modification request may be accepted or needs to be rejected in case of resource limitations (typically used for admission control of GBR traffic). It may also be used to decide which existing bearers to pre-empt during resource limitations.
The range of the ARP priority levels is 1 to 15 with 1 as the highest level of priority. The pre-emption capability information defines whether a service data flow may get resources that were already assigned to another service data flow with a lower priority level. The pre-emption vulnerability information defines whether service data may lose the resources assigned to it in order to admit a service data flow with higher priority level. The pre-emption capability and the pre-emption vulnerability may be either set to “yes” or “no”.
For example, video telephony is one use case where it may be beneficial to use EPS bearers with different ARP values for the same UE. IN this use case an operator could map voice to one bearer with a higher ARP and video to another bearer with a lower ARP. In a congestion situation (e.g. at call edge) the eNB may then drop the “video bearer” without affecting the “voice bearer”. This would improve service continuity.
In the current packet core network, according to 3GPP 23.107 (table 4): value ranges for UMTS bearer service attributes, (ARP) Allocation/retention Priority is defined as a scalar with a range 1-3 which has been implemented at the Gr interface and Gn/Gp interface.
How to introduce the Evolved ARP and at the same time to avoid the backwards compatibility issues becomes a problem.
PDG-GW supporting E-UTRAN access with MME/S4-SGSN need to communicate with legacy SGSN via Gn/Gp interface to allow UE handover from 2G/2G to E-UTRAN access, and vice versa.
The legacy SGSN, GGSN, and HLR need a solution to seamlessly migrate to the Evolved ARP concept to allow introducing the services which evolved ARP is required. Again, PDN-GW and MME need a solution to communicate with legacy SGSN when come to ARP handling.